<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
		@media (min-width:300px) and (max-width: 500px){/* 宽度不超过2000px,背景颜色就是红色 */
			body{
				background: red;
			}
			#a{
				opacity: 0;
			}
		}
		button{
			opacity: 0;
		}
	#a{
		height:200px;
		width:200px;
		background:#000;
	}
	
	</style>
</head>
<body>
<button>fdfdsa</button>
	<div id="a">
		
	</div>
</body>
</html>